Thanks for your reply. I've opened up the player and removed the battery and connector easily. I've contacted a couple of sites already asking if the battery is suitable for my player, but they have responded that they don't have suitable batteries for my player.
Several batteries state they are suitable for the S638red which is the 8gb version. I assume it would would use the same battery, but the numbers are different. I will contact a few more companies and possibly take a punt on one.
I'll post some photos and info if I do manage to get a battery to work.